Apparatus and method for processing a corrupted frame

ABSTRACT

An apparatus and method for processing a corrupted frame. The corrupted frame is analyzed to ascertain whether the rate can be determined. If the rate can be determined, steps for identifying the rate are executed. After the rate is identified, the method scans the corrupted frame by checking the MM/TT/TM or MM/FM bits (depending on the radio configuration) to determine whether signaling is present in the frame. If the scan finds potential signaling information, the BTS informs the transcoder of the frame rate, that signaling is present in the frame and that the outer CRC failed. The transcoder attempts to recover the signaling information based upon the inner CRC and discards the bearer information. If the signaling message is spread over multiple frames, the entire frame is discarded to avoid confusion when the signaling message has to be reassembled. The entire frame is also discarded if the inner CRC fails.

REFERENCE(S) TO RELATED APPLICATION(S)

The present application claims priority from provisional application,Ser. No. 60/371,850, entitled “APPARATUS AND METHOD FOR PROCESSING ACORRUPTED FRAME,” filed Apr. 11, 2002, which is commonly owned andincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to communication systems and inparticular to an apparatus and method for processing a corrupted frame.

BACKGROUND OF THE INVENTION

Currently, when a data frame is transmitted between a Mobile Station(MS) and a Base Transceiver Station (BTS), a rate determinationalgorithm running in the BTS determines the frame rate and whether theframe is usable or corrupted. The rate determination algorithm relies onthe outcome of a traffic frame cyclic redundancy check (CRC) (alsoreferred to as an outer CRC), SER/Total Metric and Quality bits. Theactual payload of the frame itself is not considered in the ratedetermination process. All signaling messages contain a sixteen (16) bitCRC value (inner CRC) which is a stronger CRC than the outer CRCs used(typically 12, 10 and 8 bits). Currently, the inner CRC is notconsidered in determining whether the frame is corrupted. Signalingmessages are usually in the form of dim and burst (partially voice,partially signaling) when transmitted on the fundamental channel. Thesignaling message with its 16 bit CRC is usually a small portion of theframe on the reverse link. If the bit errors that cause the ratedetermination algorithm to declare an erasure are not in the signalingportion of the message, the signaling portion of the frame can berecovered from the otherwise erased frame. However, currently when bitserrors are detected in non-signaling portions of a frame, the entireframe is discarded.

FIG. 1 shows the format of a frame 100 transmitted over the DedicatedControl Channel/Fundamental Channel (DCCH/FCH). As specified inTIA/EIA/IS2000.2-A, the frame consists of Mixed Mode/Frame Mode (MM/FM)bits or Mixed Mode/Traffic Type/Traffic Mode (MM/TT/TM) bits, bearerbits, signaling bits, an outer CRC and tail bits. The signaling bitsconsist of a signaling message containing start of message (SOM) bits,message length, message body and an inner CRC. In current systems, arate determination algorithm running in the BTS checks the outer CRC todetermine whether a frame erasure has occurred. When the outer CRCindicates a corrupted frame, the entire frame is erased. As shown inFIG. 1, when there are no bits errors in the signaling message portionof the frame, the signaling message will still be erased based on theouter CRC check. This loss of signaling information can result indecreased system performance due to increased call setup time caused byincreased retransmissions and reduced handoff success.

Thus, there is a need for an improved apparatus and method forprocessing a corrupted frame.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a DCCH/FCH frame format in accordance with knownart.

FIG. 2 is a block diagram of a simplified communication system that canimplement the preferred embodiment of the present invention.

FIG. 3 is a diagram of a DCCH/FCH frame that has been transmitted over anoisy RF channel.

FIG. 4 is a flow diagram of the preferred embodiment of the method ofthe present invention.

FIG. 5 is a flow diagram of one embodiment of detecting a corruptedframe in accordance with the method of FIG. 4.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT

Referring to FIG. 2, a simplified block diagram of a communicationsystem that can implement the preferred embodiment of the presentinvention is shown. In the system 200, a MS 202 transmits a frame overthe DCCH or FCH to a BTS 204. In the BTS, the frame is processed by thedemodulator as known in the art and sent to the decoder for processing.In accordance with the present invention, the decoder runs a method forprocessing a corrupted frame that determines whether there is apotential for signaling information to be recovered from the corruptedframe. If there is potential signaling information, the decoder signalsa transcoder 210 to attempt recovery of the signaling information fromthe corrupted frame. Details of the MS 202, BTS 204 and transcoder 210are not particularly relevant to the present invention and are notfurther discussed herein.

The method of the present invention scans corrupted frames for possiblesignaling messages. The success of recovering signaling messages relieson the probability that the first few bits of the frame (MM/FM orMM/TT/TM) are uncorrupted along with the signaling message itself.Referring to FIG. 3, a DCCH/FCH frame that has been transmitted over anoisy RF channel is shown. As shown, possible bit errors (designated by“X”) exist in the primary bits, zero fill portion of the signaling bits,CRC and the tail bits of the frame. Any “X” in the frame results in anouter CRC failure. As stated previously, in current rate determinationmethods, the entire frame, including the signaling message, would bediscarded based on failure of the outer CRC. In the method of thepresent invention, the inner CRC is checked and if it passes (no errorbits found), recovery of the signaling message is possible. Because theouter CRC failed, the bearer bits are discarded.

In accordance with the present invention, a frame that is declared anerasure (corrupted frame) based on the outer CRC failure is analyzed toascertain whether the corrupted rate can be determined. If the rate canbe determined, steps for identifying the rate are executed. After thecorrupted rate is identified, the method scans the corrupted frame bychecking the MM/TT/TM or MM/FM bits (depending on the radioconfiguration) to determine whether signaling is present in the frame.If the scan finds possible signaling information, the BTS 204 informsthe transcoder 210. The vast majority of signaling messages fit within asingle frame. In the event the signaling message is spread over multipleframes, the preferred embodiment is to discard the entire frame to avoidconfusion when the signaling message has to be reassembled at thetranscoder 210. In some cases, the BTS could falsely detect a signalingmessage which could lead to complications during re-assembly. The vastmajority of signaling messages fit within one frame. Thus, in thepreferred embodiment, the method of the present invention, does notattempt to recover signaling messages longer in duration than one frame.This is done in order to avoid unnecessary complication of the method.

Turning now to FIG. 4, the preferred embodiment of the method forprocessing a corrupted frame is shown. The method begins at step 402,where the BTS 204 detects that it has received a corrupted frame. Atstep 404, the method determines whether the frame is so corrupted thatthe rate cannot be determined. If the answer is yes, the method reportsthe frame rate as erasure (which signals that the entire frame should bediscarded) (step 418) and the method ends (step 420). If the answer isno, the method determines the rate of the corrupted frame (step 406). Inone embodiment, the method uses the steps of FIG. 5 to determine therate. Referring to FIG. 5, the method first normalizes subrate symbolerror rates (SERs) of the frame to full rate (step 502). Then, themethod determines the rate of the corrupted frame to be the rate withthe lowest normalized SER (step 504). In an alternate embodiment, themethod can implement the method for rate determination as disclosed inFIG. 2 and the corresponding description of U.S. Pat. No. 5,878,098,entitled “Method and Apparatus for Rate Determination in a CommunicationSystem, issued on Mar. 2, 1999 to the owner of the present patentapplication (Motorola, Inc.), which is herein incorporated by reference.

Referring back to FIG. 4, after the rate of the corrupted frame has beendetermined, the method determines whether the radio configuration of theframe is radio configuration one (RC1) or radio configuration three(RC3) (step 408). If the radio configuration is one or three, at step410 the method determines whether the rate of the corrupted frame(determined at step 406) is full. If the rate is full, the methoddetermines whether the MM/TT/TM bits are equal to either of bitsequences 1000, 1001, 1010 or 1011. If the MM/TT/TM bits are equal toeither of the bit sequences, there is potential signaling information inthe corrupted frame and the method proceeds to step 414. At step 414,the method determines whether the signaling message is contained withina single frame. In the preferred embodiment, the method determineswhether the start of message (SOM) equals “1” and whether the signalingmessage length is less than the frame length. If the answer is yes, themethod reports the rate of the frame as full, that there is signaling inthe frame; and that the outer CRC failed (step 416). Then, the methodends at step 420.

Referring back to step 410, because signaling is present only in fullrate frames for RC1 and RC3, if the rate of the corrupted frame is notfull, the method reports the frame rate as erasure (step 418) and ends(step 420). Referring back to step 412, if MM/TT/TM is not equal to1000, 1001, 1010 or 1011 indicating that no signaling is present in theframe, the method reports the frame rate as erasure (step 418) and ends(step 420). Referring back to step 414, if the signaling message is notcontained within a single frame, the method reports the frame rate aserasure (step 418) and ends (step 420).

Referring back to step 408, if the radio configuration is not RC1 orRC3, at step 422, the method determines whether the radio configurationis RC2 or RC4. If the radio configuration is neither RC2 nor RC4, themethod reports the frame rate as erasure (step 418) and ends (step 420).If the radio configuration is either RC2 or RC4, the method determineswhether the rate of the corrupted frame is full (step 424). If the rateof the corrupted frame is full, the method determines whether the MM/FMbits are equal to either of bit sequences 10000, 10001, 10010, 10011 or11000. If the answer is no, the method reports the frame rate as erasure(step 418) and ends (step 420). If the MM/FM bits are equal to either ofthe bit sequences, there is potential signaling information in thecorrupted frame and the method proceeds to step 414. At step 414, themethod determines whether the signaling message is contained within asingle frame. If the answer is yes, the method reports the rate of theframe as full, that there is potential signaling in the frame; and thatthe outer CRC failed (step 416). Then, the method ends at step 420.

Referring back to step 424, if the rate of the corrupted frame is notfull, the method determines whether the rate is half (step 428). If therate is half, the method determines whether the MM/FM bits are equal toeither of bit sequences 1000, 1001, 1010 or 1110 (step 430). If theMM/FM bits are not equal to either of the bit sequences, the methodreports the frame rate as erasure (step 418) and ends (step 420). If theMM/FM bits are equal to either of the bit sequences, there is signalinginformation in the corrupted frame and the method proceeds to step 414.At step 414, the method determines whether the signaling message iscontained within a single frame. If the answer is yes, the methodreports the rate of the frame as half, that there is signaling in theframe; and that the outer CRC failed (step 416). Then, the method endsat step 420.

Referring back to step 428, if the rate of the corrupted frame is nothalf, the method determines whether the rate is quarter (step 432). Ifthe rate is not quarter, the method reports the rate as erasure (step418) and the method ends (step 420). If the rate is quarter, the methoddetermines whether the MM/FM bits are equal to either of bit sequences100 or 101 (step 434). If the MM/FM bits are not equal to either of thebit sequences, the method reports the frame rate as erasure (step 418)and ends (step 420). If the MM/FM bits are equal to either of the bitsequences, there is potential signaling information in the corruptedframe and the method proceeds to step 414. At step 414, the methoddetermines whether the signaling message is contained within a singleframe. If the answer is yes, the method reports the rate of the frame asquarter, that there is potential signaling in the frame; and that theouter CRC failed (step 416). Then, the method ends at step 420.

In accordance with the preferred embodiment of the present invention,the information reported at steps 416 and 418 of FIG. 4 is used by thetranscoder 210 of FIG. 2 in determining how to treat the received frame.Instead of treating the entire frame as an erasure (i.e. discarding theentire frame) because there are bit errors in portions of the frameother than the signaling information, the transcoder 210 can use theinformation at step 416 to attempt to recover the frame's signalingcontent, discard only primary or secondary traffic information (becausethe outer CRC failed) and instruct the MS 202 to increase the power forsubsequent frame transmissions. The transcoder 210 uses the informationat step 418 to discard the entire frame and instruct the MS to increasethe power for subsequent frame transmissions. Discarded primary andsecondary traffic frames are currently handled as erasures by thevocoder.

Preferably, the method of the present invention is stored in memory ofthe BTS 204 and transcoder 210 and runs on any computer ormicroprocessor commonly known in the art. The method results inpotentially recovering signaling content in a corrupted frame that wouldotherwise be erased. The method improves the probability that asignaling message will be successfully transmitted to the BTS on thefirst attempt. This improvement can result in reduced call setup timedue to fewer retransmissions, improved handoff success, less droppedcalls and more successful originations. While the invention may besusceptible to various modifications and alternative forms, a specificembodiment has been shown by way of example in the drawings and has beendescribed in detail herein. However, it should be understood that theinvention is not intended to be limited to the particular formsdisclosed. Rather, the invention is to cover all modifications,equivalents and alternatives falling within the spirit and scope of theinvention as defined by the following appended claims.

1. A method of processing a corrupted frame comprising the steps of:detecting that a corrupted frame has been received; determining a rateof the corrupted frame; determining a radio configuration of thecorrupted frame; using the radio configuration of the frame, rate of theframe and predetermined bits in the frame, determining whether signalinginformation is present in the corrupted frame; when signalinginformation is present in the corrupted frame, reporting the rate, thatthere is signaling present in the frame and that a frame cyclicredundancy check failed.
 2. The method of claim 1 wherein the step ofdetermining a rate of the corrupted frame comprises the steps of:determining whether the frame is so corrupted that a rate cannot bedetermined; and when the rate is not so corrupted that the rate cannotbe determined, normalizing subrate system error rates of the frame tofull rate; determining that the rate of the corrupted frame is equal toa lowest normalized subrate system error rate.
 3. The method of claim 2wherein the step of determining whether signaling information is presentin the corrupted frame comprises determining whether the radioconfiguration is one of radio configuration one and radio configuration3.
 4. The method of claim 3 wherein the step of determining whethersignaling information is present in the corrupted frame furthercomprises: determining whether the rate of the corrupted frame is full;when the rate of the corrupted frame is full, determining whether thepredetermined bits equal one of 1000, 1001, 1010 and 1011; and when thepredetermined bits equal one of 1000, 1001, 1010 and 1011, determiningthat there is signaling information in the frame.
 5. The method of claim4 wherein when the rate of the corrupted frame is not full, the methodcomprises: determining that signaling is not present in the corruptedframe; and reporting that the rate is erasure.
 6. The method of claim 4wherein the predetermined bits do not equal one of 1000, 1001, 1010 and1011, the method comprises: determining that signaling is not present inthe corrupted frame; and reporting that the rate is erasure.
 7. Themethod of claim 2 wherein the step of determining whether signalinginformation is present in the corrupted frame comprises determiningwhether the radio configuration is either radio configuration two orradio configuration four.
 8. The method of claim 7 wherein the step ofdetermining whether signaling information is present in the corruptedframe further comprises: determining whether the rate of the corruptedframe is full; when the rate of the corrupted frame is full, determiningwhether the predetermined bits equal one of 10000, 10001, 10010, 10011and 11000; and when the predetermined bits equal one of 10000, 10001,10010, 10011 and 11000, determining that there is signaling informationin the frame.
 9. The method of claim 7 wherein the step of determiningwhether signaling information is present in the corrupted frame furthercomprises: determining whether the rate of the corrupted frame is half;when the rate of the corrupted frame is half, determining whether thepredetermined bits equal one of 1000, 1001, 1010 and 1110; and when thepredetermined bits equal one of 1000, 1001, 1010 and 1110, determiningthat there is signaling information in the frame.
 10. The method ofclaim 7 wherein the step of determining whether signaling information ispresent in the corrupted frame further comprises: determining whetherthe rate of the corrupted frame is a quarter; when the rate of thecorrupted frame is a quarter, determining whether the predetermined bitsequal one of 100 and 101; and when the predetermined bits equal one of100 and 101, determining that there is signaling information in theframe.
 11. A method of processing a corrupted frame comprising the stepsof: detecting that a corrupted frame has been received; determining arate of the corrupted frame; determining a radio configuration of thecorrupted frame; using the radio configuration of the frame, rate of theframe and predetermined bits in the frame, determining whether signalinginformation is present in the corrupted frame; when signalinginformation is present in the corrupted frame, determining whether thesignaling information is contained within a single frame; and when thesignaling information is contained within a single frame, reporting therate, that there is signaling present in the frame and that a framecyclic redundancy check failed.
 12. The method of claim 11 wherein whenthe signaling information is not contained within a single frame,reporting the rate as erasure.
 13. The method of claim 11 wherein thestep of determining a rate of the corrupted frame comprises the stepsof: determining whether the frame is so corrupted that a rate cannot bedetermined; and when the rate is not so corrupted that the rate cannotbe determined, normalizing subrate system error rates of the frame tofull rate; determining that the rate of the corrupted frame is equal toa lowest normalized subrate system error rate.
 14. The method of claim11 wherein the step of determining whether signaling information ispresent in the corrupted frame comprises determining whether the radioconfiguration is one of radio configuration one and radio configurationthree; when the radio configuration is one of radio configuration oneand radio configuration three 3, determining whether the rate of thecorrupted frame is full; when the rate of the corrupted frame is full,determining whether the predetermined bits equal one of 1000, 1001, 1010and 1011; and when the predetermined bits equal one of 1000, 1001, 1010and 1011, determining that there is signaling information in the frame.15. The method of claim 11 wherein the step of determining whethersignaling information is present in the corrupted frame comprises:determining whether the radio configuration is one of radioconfiguration two and radio configuration four; when the radioconfiguration is one of radio configuration two and radio configurationfour, determining whether the rate of the corrupted frame is full; whenthe rate of the corrupted frame is full, determining whether thepredetermined bits equal one of 10000, 10001, 10010, 10011 and 11000;and when the predetermined bits equal one of 10000, 10001, 10010, 10011and 11000, determining that there is signaling information in the frame.16. The method of claim 11 wherein the step of determining whethersignaling information is present in the corrupted frame furthercomprises: determining whether the radio configuration is one of radioconfiguration two and radio configuration four; when the radioconfiguration is one of radio configuration two and radio configurationfour, determining whether the rate of the corrupted frame is half; whenthe rate of the corrupted frame is half, determining whether thepredetermined bits equal one of 1000, 1001, 1010 and 1110; and when thepredetermined bits equal one of 1000, 1001, 1010 and 1110, determiningthat there is signaling information in the frame.
 17. The method ofclaim 11 wherein the step of determining whether signaling informationis present in the corrupted frame further comprises: determining whetherthe radio configuration is one of radio configuration two and radioconfiguration four; when the radio configuration is one of radioconfiguration two and radio configuration four, determining whether therate of the corrupted frame is a quarter; when the rate of the corruptedframe is a quarter, determining whether the predetermined bits equal oneof 100 and 101; and when the predetermined bits equal one of 100 and101, determining that there is signaling information in the frame.
 18. Amethod of processing a corrupted frame comprising the steps of:receiving a rate of a corrupted frame; receiving notification that thereis signaling in the corrupted frame; receiving notification that atraffic cyclic redundancy check failed; performing an inner cyclicredundancy check; when the inner cyclic redundancy check passes,recovering the frame's signaling content; and discarding the frame'sbearer traffic content.
 19. A storage medium having stored thereon a setof instructions which, when loaded into a microprocessor, causes themicroprocessor to perform the following steps: detecting that acorrupted frame has been received; determining a rate of the corruptedframe; determining a radio configuration of the corrupted frame; usingthe radio configuration of the frame, rate of the frame andpredetermined bits in the frame, determining whether signalinginformation is present in the corrupted frame; when signalinginformation is present in the corrupted frame, reporting the rate, thatthere is signaling present in the frame and that a frame cyclicredundancy check failed.